The presence of volatile organic compounds (“VOCs”), semi volatile organic compounds (“SVOCs”) or pesticides in subsurface soils and groundwater is a well-documented and extensive problem in industrialized and industrializing countries. Many VOC's and SVOC's are compounds which are toxic or carcinogenic, are often capable of moving through the soil under the influence of gravity and serve as a source of water contamination by dissolution into water passing through the contaminated soil. Illustrative of such organic contaminants are compounds such as trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), carbon tetrachloride, chloroform, chlorobenzenes, benzene, toluene, xylene, ethyl benzene, ethylene dibromide, methyl tertiary butyl ether, polyaromatic hydrocarbons, polychlorobiphenyls, phthalates, 1,4-dioxane, nitrosodimethyl amine, and methyl tertbutyl ether.
In many cases discharge of these compounds into the soil leads to contamination of aquifers resulting in potential public health impacts and degradation of groundwater resources for future use. Treatment and remediation of soils contaminated with VOC or SVOC compounds may be expensive, require considerable time, and in many cases be incomplete or unsuccessful. Treatment and remediation of volatile organic compounds that are either partially or completely immiscible with water (i.e., Non Aqueous Phase Liquids or NAPLs) have been particularly difficult. Also treatment of highly soluble but biologically stable organic contaminants such as MTBE and 1,4-dioxane are also quite difficult with many conventional remediation technologies. This is particularly true if these compounds are not significantly naturally degraded, either chemically or biologically, in soil environments. NAPLs present in the subsurface can be toxic to humans and other organisms and can slowly release dissolved aqueous or gas phase volatile organic compounds to the groundwater resulting in long-term (i.e., decades or longer) sources of chemical contamination of the subsurface. In many cases subsurface groundwater contaminant plumes may extend hundreds to thousands of feet from the source of the chemicals resulting in extensive contamination of the subsurface. These chemicals may then be transported into drinking water sources, lakes, rivers, and even basements of homes through volatilization from groundwater.
The U.S. Environmental Protection Agency (USEPA) has established maximum concentration limits for various hazardous compounds. Very low and stringent drinking water limits have been placed on many halogenated organic compounds. For example, the maximum concentration limits for solvents such as trichloroethylene, tetrachloroethylene, and carbon tetrachloride have been established at 5 .mu.g/L, while the maximum concentration limits for chlorobenzenes, polychlorinated biphenyls (PCBs), and ethylene dibromide have been established by the USEPA at 100 .mu.g/L, 0.5 .mu./L, and 0.05 .mu.g/L, respectively. Accordingly, there is a need for improved methods of achieving environmental remediation.
Curtin et al, Ascorbate-induced oxidation of formate by peroxodisulfate: product yields, kinetics and mechanism, Res. Chem. Intermed., Vol. 30, No. 6, pp. 647-661 (2004) discloses that the rate constant (k1′) of the reaction of persulfate with ascorbic acid to produce an active SO4− radical is 0.02, thereby indicating that ascorbic acid is ineffective to activate persulfate.
Huling et al, Groundwater Sampling at ISCO Sites: Binary Mixtures of Volatile Organic Compound and Persulfate, Ground Water Monitoring & Remediation 31, no. 2/Spring 2011/pages 72-79 disclose the use of high levels of ascorbic acid (of molar ratios of 4:1 or greater of ascorbic acid:persulfate) to preserve samples of sodium persulfate and VOCs for analytical purposes. The addition of such high proportions of ascorbic acid deactivates the persulfate without significantly reducing the amounts of VOCs (benzene, toluene, m-xylene, perchloroethylene and trichloroethylene) present in such samples, further suggesting that ascorbic acid is ineffective to activate persulfate in remedial situations.
Accordingly, it is completely unexpected that the addition of lesser proportions of ascorbic acid and/or other organic acids selected from the group consisting of formic acid, oxalic acid, lactic acid and citric acid will effectively activate persulfate such that organic contaminants contacted with such mixture will be effectively oxidized.